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As apatite is generally only a minor mineral in the soil mineral matrix, its contribution to the soil solution Ca pool is minor compared to other minerals.  If the Ca isotopes in the plant is more similar to the signature in apatite than in the soil solution, it indicates that the plant takes up Ca directly from the apatite crystal. Which is below the root scale, indicating a selective uptake via mycorrhizal hyphae colonizing apatite grains.   In an influential paper Blum *et al.* \cite{Blum_2002} applied this technique, but as in their study area, the different mineral sources did have similar Ca isotope ratios, they used the ratio between Ca and Sr instead.  Using element ratio increases the risk of fractionation. Already in 1926 Fay warned for the use of Ca/Sr ratio to trace sources of Ca \cite{fay_strontium_1926}.